In one known hydraulic control circuit for a power shovel, first to third circuit systems are respectively connected to first to third pumps and discharged oil from the third pump is made to merge into the first and second circuit systems as necessary.
A control circuit disclosed in JP1998-88627A is configured such that discharged oil from a third pump is supplied into a boom cylinder when only a boom switching valve provided to a first circuit system is switched, discharged oil from the third pump is supplied to an arm cylinder when only an arm switching valve is switched, and discharged oil from the third pump is preferentially supplied to the arm cylinder when the boom switching valve and the arm switching valve are simultaneously switched.
Specifically, the above-described control circuit includes a hydraulic accelerating valve for preferentially supplying discharged oil from the third pump to the arm cylinder. The hydraulic accelerating valve includes two pilot chambers to which a pilot pressure of the boom switching valve and a pilot pressure of the arm switching valve are respectively led, and a spring that imparts a biasing force in the same direction as the pilot pressure of the arm switching valve.
The hydraulic accelerating valve switches so as to supply discharged oil from the third pump to the boom cylinder by means of the pilot pressure of the boom switching valve overcoming the biasing force of the spring when only the pilot pressure of the boom switching valve acts, and switches so as to supply discharged oil from the third pump to the arm cylinder by means of the pilot pressure of the arm switching valve and the biasing force of the spring when only the pilot pressure of the arm switching valve acts. Further, the hydraulic accelerating valve switches so as to supply discharged oil from the third pump to the arm cylinder by means of a combined force of the pilot pressure of the arm switching valve and the biasing force of the spring overcoming the pilot pressure of the boom switching valve when the pilot pressures of both the boom switching valve and the arm switching valve act.